Conveyors and other apparatus used in the automatic handling of objects have shown remarkable progress recently. In addition to various optical and electrical detecting devices, apparatus utilizing air nozzles to detect whether the object is in a predetermined position or whether the shape of the object is correct is widely employed because of its simplicity and safety. In devices which transport or feed objects using vacuum suction, it is common to detect whether the device is operating properly through detection of a change of vacuum in a conduit leading to the suction head.
Apparatus utilizing air pressure change detection methods have found only limited use. One reason is that the transmission speed of the detection signal and the precision of detection is less than that provided by electrical detecting devices, despite such advantages of air detection devices as immunity from interference by temperature, humidity, vibration and noise conditions, and safety in use near inflammable materials.
Among the known air pressure measuring and control devices are those utilizing an air bridge circuit which uses an air detection nozzle to measure changes in air pressure for detecting the presence of an object and/or measure the distance to an object. Such apparatus functions as a proximity switch and is capable of generating on on-off signal by detecting the presence or absence of an object without contact. This apparatus operates independently of the material of the object, provides stable detection as it is insensitive to changes in absolute pressure of the supplied air, and has a relatively high detection sensitivity.
However, such apparatus generally suffers the drawback of a relatively slow response speed as a result of using air pressure, particularly in detecting small pressure changes. For this reason, such apparatus is practical only in providing a signal for an on-off switching function.